Heat treatment of manganese steel.



ing metals nated the 1% Drawing.

union.

WINFIELD SCOTT POTTER, orrr'rcrsnunen, PENNSYLVANIA.

HEAT TREA'I'JQZ EENT OF MANGANESE STEEL. I

To all whom it may concern:

Be it known that I, WINFmLn Scoo'r Por- TEE, a citizen of-theUnited States, residing exact description 0 the invention, such as will enable others skilled in the art to which 5 1t appertains tomake and use the same.

The present invention relates to a method of heat treating rolled, forged and otherwise wrought shapes of manganese steel; that is to say, steel containing over 5%% of manganese, with or without silicon or alloysuch as chromium or nickel, and containing bon (for instance from 25% to 2.25% of carbon). I

The product obtained by the practice of the invention has not only the'well-known.

. temperature from the upper end of the heretoughness and strength 'of manganese steel. but, in .addition thereto, is further insured aga'inst'risk of breakage inservice for the reason that in all cases the peculiar condition of manganese steel which may be desigcrackled condition is avoided.

In accordance with the improved method of heat treatment, herein described, the manganese steel product may be given the maximum attainable combination of ulti- 'mate strength and elastic limit, together with-aductilitywhich isnearly the greatgiven i the, maximum est ductility obtainable by heat treatment; or, the manganese steel product may be attainable ductility with nearly the highest possible combination of ultimate strength and elastic limit obtainable by heatjtreatment alone.

Ihave discovered a critical range of temperatures terminating, during the heating of manganese steel of ordinary composition,

at about 825 C.850 (3., and heating, a low limit of about. 710-C.='This critical range corresponds to'the absorption say, the solid resolution by the.

having. in

(that is to mixed crystals of the metal) of thegcarbids and carbon-containing a 'loyswhich'separate ,cooling 975'C., the steel has an .ultimate strength i of about 130.000 pounds per square inch and within the mass while the mass is throughand below said critical range, and, while the mass is either cooling or being heated at temperatures below said range.

I have found thatwhen the manganese steel is'heated to atemperature at, or just Specification of Letters Patent.

to be a full, clear, and;

and not a granular fracture such as the dueto heat treatment is attained by heat any suitable proportion of carabove the said Patented me. as, 1914..

Application and April 17, 1914. Serial No. 832,527.

i above the upper limit of this critical range, the steel is givena new and very finegrained structure, and if it is then waterquenched (in order to make permanent the condition due to heating), it does not tend to rupture between the grains, when subjected to shock or strain; on the contrary, if then broken, it presents, largely or 'bn tirely, a fracture that may be called silky}? occurs in the steel in which the grain adhesion is weaker than the cohesive strength of the metal in the grains. 1 have also found 1 greatest strength of the manganese stee.

treatment at a temperature just above the critical range referred to. and that the greatest elongation or ductility of the steel is obtained by heating it at about 25 C.-

50 0., above the upper limit of the critical range. Furthermore, .I have found that when the manganese steel is heated to any adhesion between the grains, which adhew sion is about equal to the cohesive strength of the metal in the grains. I hzwe also found that when the manganese steel is heated to a temperature higher than 125 0.,

critical range, the grain adhesion is greatly decreased, and that the quality of ordinary manganese steel when heat treated. for instance, at temperatures of say 975 C.1100 C. is much inferior to the, qualities obtainable by heat, treatment at temperatures between .the upper limit of the critical range-and C.125 C. above I the critical range. For instance. wrought manganese steel. in which the critical range is passed ata temperature of 850 (3., and a I Which-sis water-quenched before the heat treatment has carried it above 875 (1. has

an ultimate strength of say 160.000 pounds 106 per square inchnand an elongation of about 55%. If heat treated at about 900 C.. the 9 steel has an ultimate strength of about 150000.

pounds per souare inch-and an elongation of about 60%. Ifheat treated at about an elongation of about 55%. When heat treated at about 1025 C., the steel has an -I call the i per square inch, and an elongation of about lVhen heat treated at temperatures above 97 5 0., the stretched manganese-steelbar usually cracks between the grains, more or less, exhibiting the state of the steel which it is n'ccessaryfor steel to resist in engineering service; 1

In the practice of my improved method of heatrtreating wrought manganese steel, the wrought product, finished as to form, is placed in a furnace. which is supplied, as nearly as practicable, with a neutral or nonoxidizing atmosphere or medium, and 1s therein heated either rapidly or slowly to a predetermined maxium temperature, dependent upon the analysis of the metal, (that is, at a higher or lower temperature in propontion to the percentage of manganese) which etermines the location of the'critical point, '(at the upper limit of the hereinbefore mentioned critical range; at which critical point the metal is comp etelytransformed into the uniform austenitic state. The maximum temperature to be selected for the heat treatment will also depend upon the stiffness or ductilitydesired in the product. For example, if a product is beingheat treated in order to impart to it the ductility suitable for cold drawing, cold stamping, cold pressing, or otherwise cold forming, where a maximum of ductility is desirable, the maximum temperature of the heat treatment will be located at about 25 C.100 (3., above the austeniteforming critical range. If, however, the maximum of strength and stiffness combined with a moderately high du'ctility'is desired, the manganese steel product is heated to, or slightly above the upper limit of the critical range. When the greatest stiffness with less than thegreatest obtainable ultimate strength and ductility is preferred, the product is heated to. a-temperature at, or slightly below, the upper'limit of the critical range.

. .In contrast to previously proposed methods 'of heat treating manganese steel, the steelfis preferably held for some time in the furna ce,.ator-about the maximum temperatiiiecn iployed in the heat treatment. a In fact, it ismily by permitting the steel to cam -ma state of equilibriumnt the maximumj temperature of the heat treatment, that thefine-grained uniform austenite state of the metal, which is both strong and tough can/be produced at, or slightly above pthe upper limit of the critical'ran 'ge. By using suitable means of avoiding surface oxidation of'the manganese steel product, I have found it possible to hold the steel in the furnace, at the maximum temperature empli'i'yed, for a period of one hour or more, without loss of grain adhesion at temperatures just below, at, or even 125" C. above the upper limit of the critical range; where as, when the temperature is, for example, 150 C; or more above the upper limitof the critical range, the product can only be held in the furnace at the maximum temperature employed for such time as is necessary to impart to the metal body a uniform temperature, for the reason that, if held longer in the furnace, the grain adhesion is found to be objectionably low, beginning at the surface of the metal body and extending V inward to adepth dependent upon the time duringv which it waslheld in the furnace after the uniform maximum temperature had been obtained.

It will, of-- course, be understood, that Lwhen the manganese steel product, in the practice of my invention,

has been brought, y the heat treatment described, to the de-' sired condition of re-solution of the separable constituents referred to, and has at tained the desired condition of equilibrium, such condition is made permanent by rapidly cooling the product to'ordinary'temperatures,-as, for instance, by quenching thiilproduct in water. i

aving thus described my invention, what Iclaim is:

1. The method of heat treating a wrought manganese steel product, comprising heating the said product to a temperature substantially as high as that of the upper limit, of the critical range for the re-sol'ut'ion'of the constituents which have separated .out from the mixed crystals of the manganese steel product, retaining the product at substantially the maximum temperature of the heat treatment until the metal of the product is brought to a state of equilibrium, and, finally, rapidly cooling the product to ordinary temperature, a w

2. The method of heat treating a wrought manganese steel product, comprising heating the said product to a temperature slightly above that of the upper limit of the critical range for the resolution of the constituents which have separated, out from the mixed crys alsof the manganese steel product, retaining the product at substantially the maximum temperature of the heat treatment until the metal of the product is brought to a state of equilibrium,.and finally,

rapidly coolingthe product to ordinary tempera-ture. a a

3. The method of heat treating a wrought manganese steel product, comprising heat mgtlie said product to a maximum temperature within a range of- C. above" the Zipper limit of the/critical ange for the re olution of the constituents which have separated out from the mixed crystals of the manganese steer roduct, retaining the product at substantially the maximum temperature of the heat treatment until the metal librium,

slightly above 825 C.850

of the product is brought to a state of equiand finally, rapidly cooling the product to ordinary temperature. 1

4. The method of heat treating a wrought manganese steel product, comprising heating the said, product to avtemperature substantially as high as 825 G.-850 0., retaining the product at substantially the maximum temperature of the heat treatment until the metal of the product is brought to a state, of equilibrium, and, finally,- rapidly cooling the product to ordinary temperature;

5. The method of heat treating a wrought manganese steel product comprising heating the said product to a temperature 0., retaining the product at substantially the maximum temperature of the heat treatment until the metal of the product is brought to a state of WINFIELD SCOTT POTTER.

Witnesses:

M. A. BILL, R. MONTGOMERY. 

